Cisco customers are currently experiencing attacks due to a new worm that is active on the Internet. The signature of this worm appears as UDP traffic to port 69 and high volumes of TCP traffic to port 135 and 4444. Affected customers have been experiencing high volumes of traffic from both internal and external systems. Symptoms on Cisco devices include, but are not limited to high CPU and traffic drops on the input interfaces. This document focuses on both mitigation techniques and affected Cisco products which need software supplied by Cisco to patch properly.

There are currently two worms that both exploit systems unpatched for MS03-026, which are referred to as Blaster and Nachi. This document focuses on mitigation techniques for Blaster and our other document focusing on Nachi mitigation techniques is located at http://www.cisco.com/warp/public/707/cisco-sn-20030820-nachi.shtml. Both documents should be considered in applying mitigation techniques to deal with these issues.

The effects of this worm can be mitigated by blocking the required ports it uses to spread itself, scan for new infections, and propagate the executable code. This document focuses on blocking the spread of the worm, either before or after your internal network is infected. This worm spreads using valid ports, blocking those ports may break existing functionality, such as file sharing, TFTP or Kerberos authentication. As with all network configurations, Cisco recommends you establish documentation of baseline traffic during normal times, and use that to make decisions about blocking ports or traffic in your network. Block ports with caution to avoid disabling functionality in your network. Brief descriptions of the normal usage of these ports is listed below.

TCP port 135 is used for the MS RPC protocol. This is often used to share files on local network segments, and rarely used to share files over WAN segments. This is the port where the initial vulnerability is exploited, initiating a sequence of events that fully infects a machine. Blocking port 135 can prevent initial infections, but may disable existing filesharing functionality within your network.

UDP port 69 is used for TFTP, often used to load new software images or configurations to networked devices. A host infected with the W32.Blaster worm opens up this port to transfer the msblast.exe file from an infected machine to a newly exploited machine. Blocking this port may prevent the spread of the worm from an already infected machine to vulnerable hosts, but may break existing TFTP functionality within your network including some implementations of Voice over IP.

TCP port 4444 is used for Kerberos authentication and Oracle9i communication. A host fully infected with the W32.Blaster worm opens a command shell on this port, allowing the machine to be controlled remotely. Blocking this port may prevent an infected machine from being used for further malicious activities, but may block existing Kerberos authentication functionality or Oracle9i implementations within your network.

TCP and or UDP ports 137, 138, 139 and 593 have vulnerabilities associated with them and may leave hosts open to exploitation, but are not currently known to be directly connected to the spread of the W32.Blaster worm. Cisco recommends that any unneeded ports, particularly those with known vulnerabilities associated with them, should be blocked both inbound and outbound at edge networks to prevent their remote exploitation.

NetFlow can be a powerful tool to help identify infected hosts. Netflow must be enabled on an interface with the command ip route-cache flow. The following example shows infected hosts attempting to infect random systems on a destination port of 135, which shows in the output as 0087. Port 69 is 0045, and port 4444 is 115c.

MLS statistics can help track down infected hosts. NetFlow should be enabled in full flow to see source and destination ports, as in the following example, which shows traffic sourced from infected hosts, attempting to infect random systems on destination port tcp 135.

Overall network symptoms may manifest as increased load on firewalls, routers and switches due to increased traffic. You may see instability in networks due to increased load. The traffic load generated by this worm is high, but appears to have stabilized after the first 24 hours of infection.

Unexplained network failures may be due to filtering or blocking legitimate services with filters which are too generic -- if devices such as routers or IP phones appear to not boot, please check that they still have access to a TFTP server. These devices are not vulnerable to the W32.Blaster worm, but may depend on open TFTP functionality when they boot to load software or configuration files.

To determine if a product is vulnerable, review the list below. If the software versions or configuration information are provided, then only those combinations are vulnerable. This is a list of appliance software which needs patches downloaded from Cisco:

Cisco Secure ACS Solution Engine, also known as the Cisco Secure ACS Appliance

Software version 3.2.1 is affected. CiscoSecure ACS Solution Engine Hotfix KB824146, version 3.2(1.20) will resolve this vulnerability by both applying the patch from Microsoft MS03-039, which supercedes patch MS03-026, and adds additional security measures for the underlying operating system by disabling the following ports: TCP/UDP 137,138, 445.

To verify which version is installed, and the existence of any hotfixes or patches on your Cisco Secure ACS Solution Engine, check the System Configuration menu, then select the Appliance Upgrade Status item.

Where Cisco provides the operating system bundled with the product, Cisco is offering free software patches to address these vulnerabilities for all affected customers. Customers may only install and expect support for the feature sets they have purchased.

Customers with service contracts should contact their regular update channels to obtain any software patch containing the feature sets they have purchased. For most customers with service contracts, this means that patches should be obtained through the Software Center on Cisco's Worldwide Web site at http://www.cisco.com/tacpage/sw-center/.

Customers whose Cisco products are provided or maintained through a prior or existing agreement with third-party support organizations such as Cisco Partners, authorized resellers, or service providers should contact that support organization for assistance with obtaining the free software patch(es).

Customers who purchased directly from Cisco but who do not hold a Cisco service contract, and customers who purchase through third party vendors but are unsuccessful at obtaining fixed software through their point of sale, should obtain fixed software by contacting Cisco Technical Support using the contact information listed below. In these cases, customers are entitled to obtain a patch to a later version of the same release or as indicated by the applicable row in the Software Versions and Fixes table (noted above).

This section is focused on mitigation techniques for the W32.Blaster worm using existing Cisco products in your network. These techniques should be applied both inbound and outbound at the edge of network segments if it is determined they will not affect existing network functionality. Affected systems will still be infected and able to spread within contained sections of the network, therefore it is recommended that all affected servers be patched according to Microsoft's recommendations.

Although each of these examples show how to block all affected ports, it may not be necessary to block all ports. If you have no infected hosts within your network, it may be acceptable to only block port 135 at your network edge, this would prevent infection from outside your network without impeding existing TFTP and Kerberos services. Using NetFlow to identify normal traffic flow on your network will aid you in applying these mitigation techniques with the least impact.

Note: The worm will attempt to send packets to random IP addresses, some of which may not exist. When that occurs, the router will reply with an "ICMP unreachable" packet. In some cases, replying to a large number of requests with invalid IP addresses may result in degradation of the router's performance. To prevent that from occurring, use the following command:

Caution: Common network configurations, such as certain types of tunnel structures, require the use of "ip unreachables". If the router must be able to send "ICMP unreachable" packets, you can rate limit the number of replies using the following command:

Router(config)# ip icmp rate-limit unreachable <millisecond>

Beginning with Cisco IOS Software Release 12.0, the default rate limiting is set to two packets per second (500 ms), a value of 2000 ms is commonly used.

Receive ACL Feature—On a Cisco 12000 (GSR) series router, packets destined to the router's ip addresses are "punted" to the gigabit route processor (GRP) for processing. In order to protect the GRP, receive ACLs (rACLs) can be applied. rACLs filter traffic destined to the GRP and only traffic explicitly permitted is processed by the GRP, denied traffic is dropped. In general, rACLs do not affect transit traffic (traffic flowing through a router), only traffic destined to the router itself.

rACLs are an extremely effective countermeasure for mitigating the effects of excessive attack traffic destined to the GRP. For more information please refer to GSR: Receive Access Control Lists.

Cisco recommends the use of IOS ACLs on the Cisco Catalyst 4000 with a Sup3 and Hybrid and Native configurations of the Cisco Catalyst 6500, however a VACL configuration example is provided for your convenience. Additionally, the use of "no ip unreachables" is recommended.

Caution: As when making any configuration change, use caution when using VACLs in conjunction with IOS ACLs. Be aware that VACLs apply to all traffic within the VLAN, regardless of direction.

Apply the IOS ACL on switch virtual interfaces (SVIs), which are Layer 3 interfaces to VLANs; on physical Layer 3 interfaces; and on Layer 3 EtherChannel interfaces in both the inbound and/or outbound direction. Ensure 'no ip unreachable' is configured on the interface.

Apply the IOS ACL to Layer 2 interfaces on the switch only if an IOS ACL is not also applied to the input of a Layer 3 interface (an error message is generated upon attempts to do so). For Layer 2 interfaces the IOS ACL is supported on the physical interfaces only and not on EtherChannel interfaces. It can be applied on the inbound direction only.

The default behavior of the PIX is to block traffic from lower security level interfaces (OUTSIDE) to higher security level interfaces (INSIDE) unless the affected ports and protocols have been explicitly permitted by an access-list or conduit.

The W32.Blaster worm is due to launch TCP SYN attacks against windowsupdate.com, first starting on the 16th of August 2003.

The packets generated by the infected hosts will be destined to the http port (TCP/80) of the IP address that is resolved as windowsupdate.com. The source addresses will be spoofed to random IP addresses from the same B class as the infected host. Therefore, deploying anti-spoofing techniques may help to mitigate the effects of the DoS attack.

The two common anti-spoofing techniques available on Cisco routers are Unicast Reverse Path Forwarding (Unicast RPF) and access-lists.

When Unicast RPF is enabled on an interface, the router will examine all packets received on that interface and will make sure that the source address and the source interface match the interface on which the packet is received, otherwise the packet will be dropped. Refer to the Configuring Unicast Reverse Path Forwarding document for more information about Unicast RPF.

Deploying access-lists for anti-spoofing needs the explicit knowledge of the networks that may legitimately appear as source addresses on a particular interface. Configuring the ip access-group <acl> in command on the interface with an access-list that is permitting all authorized networks and denying the rest of the IP address space will drop the spoofed packets on that interface.

For example, on a corporate router that is connected to the internal network of 192.168.1.0/24 via fastethernet0/0, the following access-list can be used for dropping spoofed packets that are originated from this internal network:

This is a final advisory. Although Cisco cannot guarantee the accuracy of all statements in this advisory, all of the facts have been checked to the best of our ability. Cisco does not anticipate issuing updated versions of this advisory. Should there be a significant, material change in the facts, Cisco may update this advisory.

A stand-alone copy or paraphrase of the text of this security advisory that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.